1. Field of the Invention
This invention relates to an improved process for the production of sulfuric acid from waste sulfuric acid, especially waste sulfuric acid obtained as a result of solubilizing treatment of TiO.sub.2 -containing materials with sulfuric acid. This invention is especially concerned with the recovery of iron sulfate (ferrous sulfate) of low water content from ferrous sulfate of higher water content, e.g., green salt. This invention is particularly concerned with a process in which waste sulfuric acid is added to green salt having a high water content of crystallization, the ferrous sulfate is dehydrated and ferrous sulfate of low water content is filtered off to leave therebehind a diluted filtrate of waste sulfuric acid wherein a partial stream of said filtrate is recycled to the dehydration step together with waste sulfuric acid. The recovered iron sulfate is thermally decomposed and sulfuric acid is manufactured from the SO.sub.2 -containing gases.
2. Discussion of the Prior Art
The solubilizing treatment of TiO.sub.2 -containing raw materials with sulfuric acid results in waste products which consist of waste sulfuric acid and, during the treatment of ilmenite, of the so-called green salt. Waste acid contains up to about 21% by weight of free sulfuric acid and metal sulfates, mainly iron sulfate. The solid green salt consists of ferrous sulfate having a high content of water of crystallization and contains up to about 90% by weight FeSO.sub.4. 7H.sub.2 O. A recovery of the raw materials is desirable for ecological reasons.
A direct processing of the green salt to effect a thermal dissociation into iron oxide and SO.sub.2, which is processed to form sulfuric acid, is highly expensive because the dissociation must be effected at high temperatures and the high content of water of crystallization in the green salt must be heated to that temperature. Besides, the moisture must be removed from the released gases if they are to be subjected to dry catalysis.
Similar remarks are applicable to the processing of the waste acid by thermal dissociation because the sulfuric acid concentration is very low. For this reason the concentration of such waste acid is increased before they are dissociated.
It is known to conduct the waste acid which becomes available as a result of the solubilizing treatment of TiO.sub.2 -containing materials with sulfuric acid directly into the thickening zone of a crystallizer, in which concentrated sulfuric acid having a sulfuric acid concentration of 60-65% is added to increase the sulfuric acid concentration to 50-55%, and then to withdraw and filter off the ferrous sulfate, which has formed and settled and has a low content of water of crystallization, to treat the aqueous filtrate and the overflow effluent from the crystallizer, which contain 50-55% sulfuric acid, by means of an immersion burner in an evaporator to increase the sulfuric acid concentration to 60-65%, to conduct a partial stream of that acid of increased concentration into the crystallizer, and to increase the concentration of the other partial stream still further and then to re-use the latter partial stream for the solubilization of the TiO.sub.2. The solid ferrous sulfate (FeSO.sub.4.H.sub.2 O) which has been filtered off and has a low content of water of crystallization is then processed to form sulfuric acid (German Patent Specification No. 957,473). In that process, acid must be circulated at a very high rate in order that the mixing of the waste solid, which contains about 20% sulfuric acid, and of the more highly concentrated, circulated acid, which contains 60-65% sulfuric acid, results in mixed acids having a sulfuric acid concentration of 50-55%. Besides, solid green salt is not added to the process.
It is an object of the invention to enable an economical and ecologically satisfactory processing of the waste acid and of the solid ferrous sulfate, which has a high content of water of crystallization, which become available as a result of the solubilization of TiO.sub.2.